hook.php

  function processDatamap_postProcessFieldArray ($status, $table, $id, &$fieldArray, $parentObject) {
		if($table == 'sys_file_collection' && in_array($status, array('update', 'new'))) {
			if(array_key_exists('uploadfile', $fieldArray)) {
				$uploadFolder = $GLOBALS['TCA'][$table]['columns']['uploadfile']['config']['uploadfolder'];
				$uploadedImages = explode(',', $fieldArray['uploadfile']);
				unset($fieldArray['uploadfile']); // Don't want it to get persistsed as it serves a dummy

				/** @var $fileCollectionRepository TYPO3\CMS\Core\Resource\FileCollectionRepository */
				$fileCollectionRepository = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance('TYPO3\\CMS\\Core\\Resource\\FileCollectionRepository');
				$fileCollectionObject = $fileCollectionRepository->findByUid($id);
				$fileCollectionObject->loadContents();

				/** @var $resourceFactory  \TYPO3\CMS\Core\Resource\ResourceFactory */
				$resourceFactory = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance('TYPO3\\CMS\\Core\\Resource\\ResourceFactory');
				/** @var $indexerService \TYPO3\CMS\Core\Resource\Service\IndexerService */
				$indexerService = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance('TYPO3\\CMS\\Core\\Resource\\Service\\IndexerService');
				$indexerService->setFactory($resourceFactory);
				// run indexing of every storage
				$storageRecord = $GLOBALS['TYPO3_DB']->exec_SELECTgetSingleRow('*', 'sys_file_storage', 'deleted = 0 AND uid = 4');

				foreach ($uploadedImages as $image) {
					$fileObject = $resourceFactory->getFileObjectFromCombinedIdentifier('4:' . $image); #retrieveFileOrFolderObject($uploadFolder . DIRECTORY_SEPARATOR . $image);
					$indexedFileObject = $indexerService->indexFile($fileObject);
					$fileCollectionObject->add($indexedFileObject);

				}
				$fileCollectionObject->persist(); // Nothing is persisted, no file objects are saved to the collection
			}
		}
	}